The preparation of hydrocarbons from a H.sub.2 /CO mixture by contacting this mixture at elevated temperature and pressure with a catalyst is known in the literature as the Fischer-Tropch hydrocarbon synthesis. Catalysts suitable for this purpose are catalysts comprising cobalt supported on a carrier. Such catalysts can be prepared by contacting particles of a porous carrier for a considerable period of time with a solution of a cobalt compound, subsequently removing the solvent and calcining and activating the composition obtained. In this manner are generally obtained catalyst particles in which the cobalt is distributed homogeneously over the carrier material, viz. at each point of the catalyst particle the cobalt concentration present is virtually the same.
An investigation into the use of catalysts comprising cobalt supported on a carrier for the preparation of hydrocarbons from H.sub.2 /CO mixtures has recently shown that catalysts in which the cobalt is inhomogeneously distributed over the carrier material show a higher C.sub.5.sup.+ selectivity than similar catalysts in which the cobalt is distributed homogeneously over the carrier, provided that said inhomogeneous distribution meets certain requirements. In order to assess the inhomogeneity of the cobalt distribution over the catalyst particles, the latter are regarded as being composed or a kernel surrounded by a peel, the kernel being of such a shape that at every point of the kernel perimeter the shortest distance (d) to the perimeter of the peel is the same and that d is equal to all catalyst particles under consideration and has been chosen such that the quantity of cobalt present in the total peel volume (.SIGMA.V.sub.p) is 90% of the quantity of cobalt present in the total volume of the catalyst particles under consideration (.SIGMA.V.sub.c). For determining .SIGMA.V.sub.p, the "electron microprobe analysis" method can very suitably be used. It was found that the catalysts having an inhomogeneous cobalt distribution show no significant improvement in C.sub.5.sup.+ selectivity relative to catalysts with homogeneous cobalt distribution, unless the inhomogeneous cobalt distribution is such as to meet the requirement (.SIGMA.V.sub.p /.SIGMA.V.sub.c)&lt;0.85. Since the investigation has further shown that the catalysts have higher C.sub.5.sup.+ selectivities according as their (.SIGMA.V.sub.p /.SIGMA.V.sub.c) quotients are lower, the preferred catalysts for use on a technical scale are those having low (.SIGMA.V.sub.p /.SIGMA.V.sub.c) quotients, and in particular those having quotients (.SIGMA.V.sub.p /.SIGMA.V.sub.c).ltoreq.0.55.
The present patent application relates to a process for the preparation of such catalysts by immersing a porous carrier once or several times in a solution of a cobalt compound, removing the liquid from the composition after each immersion and finally calcining and activating the composition. It has been found that the quotient (.SIGMA.V.sub.p /.SIGMA.V.sub.c) of the catalysts prepared in this manner is largely dependent on the viscosity measured at 60.degree. C. (v in cS) and the temperature (T in .degree.K.) of the solution, and the immersion time (t in seconds), and that in order to prepare catalysts having a quotient (.SIGMA.V.sub.p /.SIGMA.V.sub.c).ltoreq.0.55, during the immersion the following relation between v, T and t should be satisfied: (log v/t.times.T).times.10.sup.4 .gtoreq.1.